1. Field of the Invention
The present invention relates to an apparatus for reading images and its method, in particular, an apparatus for reading color images and its method.
2. Description of the Related Art
Many apparatuses for reading color images have been developed in recent years. The typical image reading apparatus is an apparatus called the color image scanner.
Various constitutions have been devised for reading color images. For example, Kokai Koho (Publication of Unexamined Patent Application) No. JP-A-10-51649 disclosed a color scanner comprising three color, i.e., red (R), green (G) and blue (B), CCD line image sensors on which reflecting lights from the document surface are focused to form each color's image as a carriage equipped with a light source travels.
This scanner repeatedly scans the document with the three CCD sensors and outputs color converted image data for each scan. As the image is read while the carriage is travelling, the scanner had a problem that a slightest discrepancy between different color images being read can cause color distortions when color images are overlaid to recreate natural color images.
In order to solve this problem, the scanner disclosed by said Koho is equipped with a pattern consisting of diagonal lines within the reading range of the CCD sensors and reads said diagonal line pattern as well as the document image for each color using the CCD sensors. Thus, the scanner is capable of detecting any positional errors of the pixels in the secondary scanning direction from the actually read image of the diagonal pattern. The scanner corrects the pixel position error detected for each color during the reproduction process of the color image so that the pixels of each color element can be accurately overlaid, thus to provide a color image without any color distortion.
However, the scanner disclosed by said Koho requires that the travelling speed of the carriage be kept constant while reading the image using the travelling carriage. The color correction procedure mentioned above is to compensate for any color shift that can occur despite the effort of maintaining a constant carriage speed. In order to maintain a constant carriage travel speed, a high accuracy is required for the mechanical system, which results in increasing the manufacturing cost of the scanner.
In order to provide a less expensive scanner, a new type of scanner has been developed in recent years which reads the image using a stationary carriage in lieu of a travelling carriage described above. This new type of carriage stops temporarily after moving in the secondary scanning direction a distance corresponding to a line of image parallel to the main scanning direction, i.e., parallel to the lines the photodiodes of the CCD sensors are arranged and reads the image line by line by means of the RGB color CCD sensors when the carriage is stationary. Such a type of scanner reads a line of image in the main scanning direction when the CCD sensors are stationary, so that it has an advantage that there is no need for maintaining a constant carriage travelling speed with a great accuracy.
However, this type of scanner that reads the image incrementally with the RGB color CCD scanners as it travels and stops for each line of image may cause a problem that the image reading starts when the carriage has not yet completely stopped due to the inertia of the carriage's weight or a slip of the timing belt or other mechanical components.
More specifically, if the scanner is to read the image in the order of R, G and B, the R color CCD sensor starts to read one line of image as soon as the carriage's motion for one line is completed, but the reading may start before the carriage completely stops due to the inertia or the mechanical slip mentioned above. On the other hand, the readings by the G and B color CCD sensors are done after the R color CCD sensor has completed its reading. Therefore, it is quite likely that the carriage becomes completely still during the reading of the R color CCD sensor. As a result, the positions of the pixels of the line read by the G and B color CCD sensors may be offset from the positions of the pixels read by the R color CCD sensor.
In such a case, if the RGB color CCD sensors are all reading a solid color area, a minor position error does not matter as the RGB color CCD sensors still read the same colors respectively and overlaying of the scanned image data reproduce the image of the same original color. However, if the scanner is reading a color boundary area, the G and/or B color CCD sensors read different color pixels across the color boundary, different from the color pixels the R color CCD sensor reads. Therefore, overlaying of color images read independently by different color sensors may cause color shifts and may not reproduce the actual document color in the color boundary area.
In order to avoid this color shift phenomenon, one may attempt to improve the machining accuracy of the mechanical components to minimize the slipping of the mechanical system such as a timing belt. Alternatively, one may attempt to take a longer lead time between the instruction for carriage stop and the instruction for image reading start, so that the image reading starts only after the CCD sensors on the carriage have completely stopped. This may improve the color shift to a certain degree.
However, increasing the machining accuracy of the mechanical system leads to the cost increase and cancels the intended cost reduction effect by means of reading the image while the carriage is at still. Moreover, the slip of the mechanical system is unpredictable in terms of when and where it occurs, so that it is extremely difficult to eliminate the slip completely by means of increasing the machining accuracy. On the other hand, an extra waiting time provided between the instructions to stop the carriage and to start reading the image slows down the image reading speed substantially and is undesirable from the standpoint of reducing the work cycle time.